The present invention relates to supercritical fluid chromatography and, more particularly, to an apparatus for analyzing a process stream via supercritical fluid chromatography having improved sample extraction system for drawing a sample volume from a process stream and diffusing the sample volume in a volume of supercritical fluid carrier for delivery to the analyzing system.
Supercritical fluid chromatography is an analytical technology which permits the analysis of high molecular weight samples in a rapid and efficient manner. The supercritical state is the region above a substance's critical point, which is defined as the temperature and pressure at which the liquid and vapor phases of the substance exists in equilibrium with each other and become identical, forming a single phase. Above this critical point in temperature and pressure, the fluid has a viscosity close to that of a gas but the solubility of a liquid and therefore is uniquely suitable as a mobile phase for transporting a high molecular weight compound through very small diameter tubes and conduits in an analyzer such as a chromatograph. There are several substances which are well known as potential supercritical fluid phases including ammonia, pentane, isobutane, and carbon dioxide. A supercritical fluid is commonly obtained by pressurizing and heating a substance above its critical temperature and pressure.
Supercritical fluid chromatography using capillary columns requires a very small sample volume. Therefore, the sample to be analyzed can be diluted in a much larger volume of supercritical fluid. This dilution can be readily accomplished in the laboratory by diluting the sample volume with a solvent prior to passing the diluted sample to the chromatograph column. However, to provide on-line analysis of a process stream using supercritical fluid chromatography in an efficient manner, the sample preparation process must be automated. Prior attempts at automation of the dilution/sampling process has resulted in complex valving and pumping systems to mix the sample and solvents at known ratios.